Hasn't it been observed for quite a long time that heart attacks are most dangerous the first time, and subsequently less dangerous?

Meh. Ignore me. The actual subject of the article is rather different. My aforementioned tidbit seems to me to be more of a full body strengthening trend, rather than this observation that heart disease strengthens just the heart tissue. (Though I wouldn't be surprised if, in general, blood vessels are more resilient too as a result of heart disease)